firmness vs obstinacy

firmness

noun
  • strictness; mental strength 

  • security; steadfastness; good grip 

  • physical durability; rigidness (of material) 

obstinacy

noun
  • The state, or an act, of stubbornness or doggedness. 

How often have the words firmness and obstinacy occurred in a corpus of books? (source: Google Ngram Viewer )